A tiny flash of light was observed at the point, whenever an α-particle struck the screen.

Observation:

Most of the alpha (α) particles passed through the gold foil without deflection.

Some of the α-particles (a small fraction) were deflected by small angles.

Very few of them were deflected nearly by 180°.

Conclusions:

Most of the space in an atom is empty as most of the α-particles passed through the gold foil un-deflected.

The positive charge is concentrated in a very small volume that repelled and deflected few positively charged α-particles. This small volume of the atom is called nucleus.

The volume of the nucleus is negligibly small as compared to the total volume of the atom.

Rutherford’s Model of Atom

Based on his experiment, Rutherford proposed:

Positive charge and most of the mass of an atom are densely concentrated in a small region (called nucleus).

Electrons revolve around the nucleus, with very high speeds, in circular paths called orbits.

Electrons and nucleus are held together in the atom by a strong electrostatic force of attraction.

Drawbacks of Rutherford’s Model

It cannot explain the stability of an atom on the basis of classical mechanics and electromagnetic theory.

If the electrons were stationary, then the strong electrostatic force of attraction between the dense nucleus and the electrons would pull the electrons towards the nucleus. Thus, it cannot explain the stability of an atom.

Rutherford’s model does not give any idea about the distribution of electrons around the nucleus (i.e., the electronic structure of the atom), and about their energy.

It cannot explain the atomic spectra.

Atomic Number and Mass Number

Atomic number (Z) = Number of protons in the nucleus of an atom

= Number of electrons in a neutral atom

Mass number (A) = Number of protons (Z) + Number of neutrons (n)

Isotopes and Isobars

Isotopes are the atoms with the same atomic number, but different mass numbers.

For example, and are the isotopes of hydrogen.

Isobars are the atoms with the same mass number, but different atomic numbers.